<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>关于默认宽度</title>
    <style>
        div {
            height: 200px;
            margin: 50px;
            border: 5px solid black;
            padding: 5px;
            background-color: gray;
        }
    </style>
</head>

<body>
    <div>你好</div>
</body>

</html>